1. Field of the Invention
This invention relates to an improved expansion shell assembly for mine roof bolts and more particularly to an expansion shell that facilitates the flow of resin into surrounding relation with the shell.
2. Description of the Prior Art
It is well known in the art of mine roof control to tension bolts anchored in bore holes drilled into the mine roof in order to reinforce the unsupported rock formation above the roof. Conventionally, a hole is drilled into the rock formation. The end of the bolt in the rock formation is anchored either by engagement of an expansion shell assembly on the end of the bolt with the rock formation, by bonding the bolt with resin to the rock formation surrounding the bore hole, or by use of both an expansion shell assembly and resin together to retain the bolt within the hole.
Examples of an arrangement utilizing both an expansion shell assembly and resin to anchor a mine roof bolt in a rock formation are disclosed in U.S. Pat. Nos. 4,419,805; 4,413,930; 4,516,885 and 4,518,292. Other examples of both an expansion shell assembly and resin to anchor a mine roof bolt are shown in U.S. Pat. Nos. 3,188,815; 4,162,133; 4,655,645 and 4,664,561.
Mechanical expansion shell assemblies for roof bolts have been used for many years without resin in the anchorage of the bolts in rock formation. Many improvements have been proposed to the configuration of various components of the expansion shell assemblies to provide better anchoring within the bore hole. When mechanical anchor assemblies are utilized in conjunction with resin bonding material, additional modifications are often made to accommodate the resin bonding material. U.S. Pat. No. 4,764,055 discloses an expansion shell assembly modified in many respects to accommodate the use of resin bonding material with the mechanical expansion shell assembly.
For many applications, it has been found desirable to use small diameter bore holes within which to install mine roof bolts. When small diameter bore holes are utilized, the power required to drill the bore hole is greatly reduced, the size and weight of the drill steel is reduced and the size and weight of the roof bolt utilized in the small diameter hole is also reduced, providing for cost savings and more efficient roof control.
It has been determined that a roof bolt with a mechanical expansion assembly can be very effectively utilized in a one inch bore hole if the mechanical expansion shell assembly is properly arranged to provide strong gripping action on the inside of the bore hole. When used with a resin system, it is important that the resin components be thoroughly mixed. The expansion shell assembly must penetrate the mixed resin to assure that the resin completely surrounds the assembly for secure bonding of the assembly to the surrounding rock formation. This is particularly important for small diameter bore holes.
A conventional expansion shell assembly includes a tapered plug threaded onto the end of a mine roof bolt and positioned within an expansion shell. The tapered plug advances downwardly on the bolt as the bolt is rotated. As the plug moves downwardly it urges the expansion leaves to expand or deflect radially outwardly to grip the rock formation surrounding the bore hole.
Generally, expansion shell assemblies are of two types. One type has a base portion in the configuration of a ring or collar to which are integrally affixed a plurality of upwardly extending expansion leaves or fingers spaced from one another and having free end portions. The leaves and ring surround the bolt. As the tapered plug moves downwardly toward the ring, the leaves bend outwardly into gripping engagement with the rock formation. Another general type of expansion shell is a bail-type shell in which two expansion leaves are supported by a bail that extends over the end of the mine roof bolt and prevents the expansion leaves from moving axially relative to the bolt.
The present invention is directed to an improved expansion shell assembly of the type having a plurality of expansion leaves integrally affixed to a collar to form an expansion shell. U.S. Pat. No. 4,764,055 discloses an expansion shell assembly representative of this general configuration. While expansion shell assemblies for anchoring mine roof bolts in bore holes are well known, there is need to improve the gripping power of the expansion shell when expanded in the bore hole of a rock formation, such as a mine roof. Particularly when used with a mixed resin system, the expansion shell must pass freely in the bore hole to penetrate the resin. The resin must flow in and around the shell so that the shell is encapsulated in the resin. Therefore, there is need to provide an expansion shell that promotes the flow of the resin in complete surrounding relation with the shell to assure secure bonding of the shell to the rock formation.